In a mixing bowl, combine soy sauce, peanut butter, brown sugar, vegetable oil, sesame oil, minced garlic, grated ginger, ground coriander, cumin, and chili powder (if using). Whisk until the mixture is smooth and well-blended.
Place the chicken thighs in a large resealable bag or shallow dish. Pour the marinade over the chicken, ensuring all pieces are coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours for maximum flavor.
Preheat your grill or grill pan over medium-high heat.
Remove the chicken from the marinade, allowing excess marinade to drip off. Discard the remaining marinade.
Place the chicken thighs on the grill and cook for 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C) and the chicken is nicely charred.
Remove the chicken from the grill and let it rest for a few minutes.
Slice the chicken into bite-sized pieces and garnish with chopped cilantro. Serve with lime wedges and your favorite peanut sauce.